The recent developments of scanning microprobe technology have made possible the detection and inspection of the topographical structure of a sample surface with sub nanometer resolution. The scanning tunneling microscope (STM) technology as disclosed in Binnig et al., U.S. Pat. No. 4,343,993, issued August 1982, is based on the tunneling currents which occur when a voltage is applied to an extremely fine conductive tip which is brought to a distance of approximately 1 nm from a surface structure. The tunneling current is generated when the conductive probe tip comes close enough to the surface so that the electron clouds surrounding the atoms in the tip and on the scanned surface overlap, thus allowing a current to flow between them when a voltage is applied. The magnitude of the tunneling current is exponentially dependent on the distance between the tip and the surface. Because of this sensitivity, the tunneling current is often used as a feedback signal to control the tip-sample separation. Additionally, the tunneling current is also a function of the conductivity of the scanned material and the surface structures. Therefore, by monitoring the variations of the tunneling current, the STM technique provides a means to detect a material structure change with a very fine resolution down to scale of individual atoms. A topographic map of a scanned surface with subnanometer resolution can be generated by scanning and monitoring the variations of the tunneling current.
One of the limitations of the scanning tunneling microscope is the requirement that the tunneling tip and the scanned surface either have to be electrically conductive or must be coated with an electrically conductive layer. Binnig, U.S. Pat. No. 4,724,318, "Atomic Force Microscope and Method for Imaging Surfaces with Atomic Resolution", issued Feb. 9, 1988, discloses an atomic force microscope (AFM) which generates a topographical image of a sample by monitoring the change of the STM tunneling current from the surface of a spring-like cantilever. The cantilever has a sharp-point fixed at one end. This sharp-point is brought to a very close proximity to the surface structure to be imaged. The atomic force between the sharp tip and the surface of the sample will cause small deflections of the cantilever corresponding to the topography of the surface. The STM in turn detects the tunneling current variations due to the deflections of the cantilever as the AFM scans the entire surface of the sample. Binnig, U.S. Pat. No. 4,724,318, by the use of an AFM, has overcome the limitation encountered in the STM technology where only a conductive surface or a conductively coated surface can be imaged.
By use of the STM or the AFM, a very high-density data storage device was proposed by topographically deforming the surface of a data storage medium to represent encoded binary bits. The encoded data is read back by scanning and detecting the bits using the STM or AFM. For example, in Quate, U.S. Pat. No. 4,575,822, "Method and Means for Data Storage Using Tunneling Current Data Readout", issued Mar. 11, 1986, a high-resolution data storage device is described in which the data is encoded as indentation, removal, or deposition of material to create a structural feature that can then be imaged. However, topographic encoding schemes are generally limited to a very low data rates because the microprobe must be servo controlled over the topographic features in order to avoid contacting and destroying the surface. Servo circuits for scanning microprobes have a bandwidth of typically 5 kHz, which is impractical for a data storage device. In addition, topographic encoding requires that the surface of the medium be exceedingly smooth so that blemishes are not misconstrued as data. This near-perfect surface must be protected from deterioration in the operating environment, for example, from oxidation or deposition of ambient contaminants. Many such schemes therefore require vacuum, protective overcoats, low temperature, inert gas flushing, or extensive filtration. Furthermore, there exists a large number of potential storage mechanisms which are non topographic and are not appropriate to the topographic readback methods disclosed in the prior art. Reliance on topographic encoding greatly limits the types of storage media that can be developed.
A method to write, read, and erase data bits which are non topographically encoded onto a storage medium by the use of an STM was disclosed by Foster et al., U.S. Pat. No. 4,916,688, "Data Storage Method Using State Transformable Materials", issued Apr. 10, 1990. Foster describes the use of a scanning tunneling microscope to selectively heat discrete areas of a state-transformable film by applying a large tunneling current to the area. Resistive heating in the film transforms the electronic structure in the area from a crystalline to an amorphous state, or from an amorphous to a crystalline state, depending on the nature of the tunneling current pulse. The change in electronic structure implies a change in conductance, work function, and band gap which can be detected by the STM as an alteration in the magnitude of the tunneling current. The effects of topography on the readback of data, e.g., the blemishes on the surface of the film, were minimized by measuring the rate of change of the current vs. the applied voltage (dI/dV) while keeping the tip/sample separation constant, or by measuring the rate of change of the tunneling current vs. the tip/sample separation (dI/dS) while keeping the voltage constant. However, this scheme, while minimizing the effect of surface topography, may be affected by the presence of material blemishes, such as oxidation or contamination, which would have to be minimized for the readback technique to operate. Furthermore, it may have difficulty with a protective overcoat because the readback technique is most sensitive to the surface layer of the film.
In some cases the material properties of the sampled surface can by themselves limit the effectiveness, i.e., the resolution of the STM application. Sakuhara et al., U.S. Pat. No. 4,837,435, "Tunneling Scanning Microscope Having Light Source", issued Jun. 6, 1989, had to apply a light to irradiate the surface to increase the conductivity in order to obtain a higher resolution in the investigation of the surface structure of the scanned material.
In the reading of discrete binary data from a storage medium using scanning microprobe techniques as disclosed in the prior art, each is limited by one or more of the following difficulties: (1) the speed of the device is limited by the bandwidth of the servo circuit that controls the motion of the tip over the topographically encoded data; (2) a near-perfect surface has to be fabricated; (3) this near-perfect surface has to be maintained under all operating conditions; (4) no protective layers can be used to protect the surface because it will affect the sensitivities of the scanning probe measurement; and (5) existing readback techniques are not sensitive to many potential bit storage mechanisms, greatly limiting the types of storage media that can be developed. Due to these problems, the development of an ultra-high density storage technology is hindered by the very high cost of both manufacturing the storage media and also maintaining the operational conditions in order to measure the highly sensitive tunneling currents for data detection.